Refrigerator for multiplier phototubes



June 25, 1968 s. A. POLLACK 3,

REFRIGERATOR FOR MULTIPLIER PHOTOTUBES Filed Sept. so, 1966 Slova A.Pollock,

INVENTOR.

AGENT.

United States Patent Office 3,389,578 Patented June 25, 1968 3,389,578REFRIGERATOR FOR MULTIPLIER PHOTOTUBES Slava A. Pollack, Palos VerdesEstates, Calif., assignor to TRW Inc., Redondo Beach, Calif., acorporation of Ohio Filed Sept. 30, 1966, Ser. No. 583,285 5 Claims.(Cl. 62-514) ABSTRACT OF THE DISCLOSURE The measurement of extremelysmall values of light flux by a multiplier phototube is limited by thethermionic dark current originating at the photocathode. This darkcurrent can be reduced to negligibly small values by refrigerating thetube. When the ambient temperature is lowered by 80-100 C., the darkcurrent drops by oneto-two orders of magnitude for the blue sensitivetubes and by four-to-five orders of magnitude for red sensitive tubes.

The conventional refrigerating procedure consists of placing the tube ina double-walled vacuum Dewar flask which is then filled with arefrigerant such as Dry Ice or liquid nitrogen. This technique iscumbersome because it restricts the use of the multiplier phototube toone position; it also requires the construction of an expensive,specially designed Dewar flask.

A simpler and more practical method of cooling consists of circulatingcold nitrogen gas around the multiplier phototube. This method permitsthe multiplier phototube to be used in a horizontal or any otherposition. Commercially available multiplier phototube assemblies, whichemploy cold nitrogen gas for cooling, house the tube in an expensivehigh vacuum metallic Dewar flask.

The present invention is based on the realization that an insignificantreduction in the dark current is gained by cooling a multiplierphototube below temperatures in the range of -50 C. to l00 C. Mostcommercially available multiplier phototubes exhibit saturation in thesignal-to-noise ratio in the aforementioned temperature range.Accordingly, it has been found unnecessary to house a multiplierphototube in an expensive high vacuum Dewar flask.

According to the invention, a refrigerator is provided which includes anouter casing of light shield material, having a thermally insulatingwindow sealed at one end thereof. The outer casing is internally linedwith an inner jacket of thermal insulation material, which in itspreferred form is made of molded polystyrene foam.

The insulation jacket houses a multiplier phototube within its interior,with an annular space or chamber provided between the jacket and themultiplier phototube. Two longitudinally spaced, angularly bentpassageways provide communication between the exterior of the outercasing and the interior of the insulating jacket. The passageways serveas inlet and outlet ports of the flow of a coolant gas into the annularspace surrounding the multiplier phototube.

The single figure of the drawing is a cross-sectioned elevational viewof a refrigerator for a multiplier phototube according to the invention.

Referring to the drawing, a cylindrical outer casing 10, made oflight-weight material, such as aluminum, has a double window 12 sealedat one end thereof. The window 12 is formed of two glass or quartz disks14 and 16 spaced from each other by a vacuum to thermally insulate theregions on opposite sides of the disks 14 and 16 from each other. Thewindow 12 may be sealed to the outer casing 10 by epoxy cement.

The end of the outer casing .10 opposite the window 12 is closed by acover 18, which may be fastened by screws 20. A multiplier phototube 22,having its base 24 disposed within an opening in the cover 18, extendslongitudinally within the outer casing 10. The phototube 22 is mountedwith its photocathode 26 spaced adjacent to the window 12 for receptionof light to be detected.

An electrical socket 28 is fastened to the tube base 24 to provideelectrical conduction means for operating potentials applied to thephototube 22 as well as for the output signal generated by thephototubev 22. A voltage divider network 30 fastened to the socket. 28and a high voltage connector 32 is used to furnish the operatingpotentials. An anode connector 34 serves as an output terminal. The highvoltage and anode connectors 32 and 34 are insulatively mounted in aperforated can 36 which surrounds the voltage divider network 30. Theperforated can 36 is fastened to the cover 18 by means of screws 38. Theperforations 40 provide cooling vents for the voltage divider network 30which generates substantial heat during tube operation.

The inner surface of the outer casing 10 is lined with a jacket 42 ofthermal insulation material, which is preferably made of moldablematerial such as polystyrene foam or the like. In addition, the insideof the cover 18 is lined with a ring 44 of similar thermal insulationmaterial which fits around the tube base 24 to thermally isolate thevoltage divider network 30.

The insulation jacket 42 is dimensioned to leave an annular space orchamber 45 surrounding the main body of the phototube 22. For coolingthe phototube 22 to its low operating temperature, the annular chamber45 surrounding the phototube 22 is filled with a coolant gas. Thecoolant gas enters the annular chamber 45 through an inlet port 46 orangularly bent passageway extending between a lower opening 48 insidethe jacket 42 and an upper opening 50 outside the outer casing 10. Theinlet port 46 may be made of stainless steel or other low thermallyconductive tubing and is bent to prevent light from leaking into theannular chamber 45.

A conduit 52 is connected between the inlet port 46 and a source 54 ofcoolant gas. The source 54 is preferably a container of liquid nitrogen56 having a. heating coil 58 immersed therein to evolve nitrogen gas.The heating of the liquid nitrogen 56 may be adjusted to cause nitrogengas to flow at a temperature between -50 C. and -100 C.

For circulating the coolant gas about the phototube 22, an outlet port60 is spaced longitudinally from the inlet port 46 in the upper regionof the gas chamber 45. The outlet port 60 is similar to the inlet port46 and may comprise a bent stainless steel tube extending between anupper opening 62 inside the insulation jacket 42 and a lower opening 64outside the outer casing 10. An open conduit 56 connected to the outletport 60 expels the coolant gas into the atmosphere. If desired theconduit 66 may be connected to the gas source 54 to return the gas tothe source 54.

A temperature monitoring device 68, such as a thermistor, is disposedwithin the gas chamber 45 adjacent to the photocathode 26 to monitor thetemperature of the latter. The temperature monitoring device 68 isbrought out to an electrical connector 70 for connection to atemperature indicating means, not shown.

The insulation jacket 42 is most conveniently made by molding it to theinside of the outer casing 10 around a removable core having the samesize as the outside dimensions of the gas chamber 45. For this purpose aliquid such as urethane Nopcofoam or Eccofoam FP, made byEmerson-Cuming, Inc., Canton, Mass, may be mixed with a catalyst to forma solid foamy material. If desired a light absorbent coating 72 such asa mat black paint or a carbon coating may be applied to the insidesurface of the insulation jacket 42 to prevent internal lightreflections. It has been found that an insulation jacket 42 formed witha thickness of about an inch or more provides satisfactory insulationfor cooling a red sensitive phototube such as a Du Mont type 6911 to -50C.

It is now apparent that the above-described refrigerator of theinvention provides a light-weight, easy to manufacture, economicalstructure for cooling multiplier phototu'bes to suificiently lowtemperatures to reduce the dark current to negligible levels. Thesimplicity of the structure is due largely to the avoidance of highvacuum construction other than the vacuum window 12.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In combination:

(a) an outer casing;

(b) a window mounted in the wall of said outer casing;

(c) a jacket of thermal insulation material lining the inner surface ofsaid outer case;

(d) a pair of longitudinally spaced angular bent passagewayscommunicating between the exterior of said outer casing and the interiorof said jacket;

(e) a multiplier phototube within the interior of said jacket and spacedfrom the latter to form an annular chamber surrounding said phototube;

(f) means for mounting said phototube with its photocathode adjacent tosaid Window for reception of light to be detected;

(g) and means for introducing a coolant gas into said chamber throughone of said passageways and for exiting said gas out of said chamberthrough the other passageway.

2. The invention according to claim 1, and further including a lightabsorbent coating on the inner surface of said insulation jacket.

3. The invention according to claim 1, wherein said jacket is made ofmolded polystyrene foam.

4. The invention according to claim 1, wherein said coolant gas meansincludes nitrogen gas at a temperature in the range of about to C.

5. The invention according to claim 1, wherein the structure in whichsaid phototube is disposed is devoid of any hermetic seals except forsaid window.

References Cited UNITED STATES PATENTS 2,948,127 8/1960 Carter 62-5l43,006,157 10/1961 Haettinger et al 62514 3,258,602 6/1966 Promish 625143,261,180 7/1966 Porter et a1. 62-45 3,293,877 12/1966 Barnes 62-5l4LLOYD L. KING, Primaty Examiner.

